Temporal and spatial evolution of coherent structures of velocity and temperature fluctuations in compressible turbulent boundary layers

Abstract

Abstract Content of image described in text. In this work, the characteristics of intense streamwise velocity and temperature fluctuation structures, including spatial and temporal properties, are investigated in compressible turbulent boundary layer flows at Mach number 2.25. To resolve the evolution process and capture large-scale structures that advect over long distances, two sets of flow field snapshots for each case, with varying domain lengths and temporal resolutions, are stored, amounting to up to 50 TB. Regions with high fluctuations are identified as structures using a clustering method, and connections between structures in consecutive flow fields are established, forming a large graph that reveals the complete temporal evolution relationship. Spatial characteristics are analysed, providing evidence for self-similarity in the logarithmic layer at this Mach number. Extra plate-like temperature structures show up extremely near the wall in the quasi-adiabatic case and their variation with different wall conditions are summarised. Conditional average associated with negative velocity fluctuations structures are also conducted, revealing a distinct lift-up of the temperature structures. Furthermore, temporal characteristics are examined, providing a foundational understanding of the structures’ kinematic behaviours. Positive velocity structures and negative temperature structures are advected faster than their corresponding counterparts with opposite signs, but all are quite close to the mean streamwise velocity except in the near-wall region. The lifetime of structures in the logarithmic layer exhibits an almost linear relationship with their height scale. Finally, the temporal and spatial evolution of the structures is described, offering a perspective to probe structure characteristics at any stage of their life cycle.